The present invention relates to ablation instruments for the treatment of atrial fibrillation and, in particular, to percutaneous instruments employing energy emitters for the ablation of tissue surrounding the pulmonary veins. Methods of ablating tissue to treat atrial fibrillation using radiant energy are also disclosed.
Atrial fibrillation is the most common cardiac arrhythmia and affects approximately 2.3 million people in the United States. It is characterized by rapid randomized contractions of atrial myocardium, causing an irregular, often rapid ventricular rate. The regular pumping function of the atria is replaced by a disorganized; ineffective quivering as a result of chaotic conduction of electrical signals through the upper chambers of the heart. Weakness, lightheadedness, fainting, heart failure, stroke and even death can result.
In 1991, Dr. James Cox developed a surgical procedure, called the Maze Procedure, to cure atrial fibrillation, It involved cutting the atrial wall into many pieces in an intricate pattern and sewing them back together. The scar tissue that formed blocked the conduction of electrical impulses and caused them to follow a pre-arranged pattern. Although this procedure was successful in curing atrial fibrillation, the difficulty of the procedure, its invasive nature and the morbidity involved prevented its widespread adoption.
Prior to the late 1990's it was believed that less invasive procedures and associated devices to cure atrial fibrillation would have to mimic the Maze procedure and create long lines of conduction block in the atrial wall. Several types of ablation devices were proposed which utilized some form of energy, generally radio-frequency (“RF”) heating to create elongated lesions that extend through a sufficient thickness of the myocardium to block electrical conduction. Many of the proposed ablation instruments are percutaneous devices that are designed to create such lesions from within the heart. Such devices are positioned in the heart by catheterization of the patient, e.g., by passing the ablation instrument into the heart via a blood vessel, such as the femoral vein. See, for example, U.S. Pat. No. 5,575,766 issued to Swartz, which discloses the use of ablation electrodes introduced into the heart via a catheter to create Maze-like lesions and U.S. Pat. No. 5,904,651 issued to Swanson, which discloses a similar electrical ablation device with an imaging element to create Maze-like lesions under visual control. However, because of the difficulty in creating long lesions that were correctly located, continuous and transmural (through the heart wall) these devices have not achieved clinical or commercial success.
In 1997 and 1998 two seminal papers by Drs. Jais and Hassaguerre identified the pulmonary veins as the primary origin of errant electrical signals responsible for triggering atrial fibrillation. By ablating the heart tissue at selected locations in or surrounding the pulmonary veins, electrical impulses from such foci of fibrillation can be blocked. In one known approach, circumferential ablation of tissue within the pulmonary veins, at the ostia (mouth) of such veins or outside of the veins has been practiced to treat atrial fibrillation. Similarly, ablation of the region surrounding the pulmonary veins as a group has also been proposed.
Several types of catheter ablation devices have recently been proposed for creating circumferential lesions to treat atrial fibrillation, including devices which employ radiofrequency, microwave or ultrasonic energy or cryogenic cooling. Regardless of the type of ablation instrument used, the desired result is a continuous circumferential lesion that isolates the foci that trigger fibrillation from the atrial tissue. Post-ablation electrical mapping is usually necessary to determine whether a circumferential lesion has been formed. If electrical conduction is still present, the encircling lesion is incomplete and the procedure must be repeated or abandoned.
Generally for the ablation instrument to be effective a clear transmission pathway is desired, e.g., a pathway from which blood has been substantially cleared. If there is too much blood between the ablation element and the target tissue, the ablation energy will be attenuated and the lesion will not be continuous and block electrical conduction.
One way to limit the amount of blood between the ablation element and the tissue is to force the ablation instrument deeper in the vein. However, if the energy application is too deep in the vein, it can result in damage to the vein and resultant narrowing of the vein, called pulmonary vein stenosis, which has serious consequences and can be life threatening.
The correct positioning of the ablation device in the heart to obtain complete lesions that will provide conduction block without damaging sensitive tissues is complicated by the fact that the standard imaging technique available to the physician for the procedure is x-ray fluoroscopy which does a poor job of imaging soft tissues such as the heart and pulmonary veins. X-ray contrast injection can be used to aid in imaging smaller arteries such as the coronary arteries but such injection is of very limited use when the entire heart or vessels as large as the pulmonary veins need to be imaged. As a result, the physician using these ablation catheters has limited ability to understand the detailed anatomy of the vein and to understand how the ablation catheter is positioned in that anatomy.
Existing instruments for cardiac ablation suffer from a variety of design limitations. In one common approach, described, for example, in U.S. Pat. No. 6,012,457 issued to Lesh on Jan. 11, 2000 and in International Application Pub. No. WO 00/67656 assigned to Atrionix, Inc, a guide wire or similar guide device is advanced through the left atrium of the heart and into a pulmonary vein. A catheter instrument with an expandable element is then advanced over the guide and into the pulmonary vein where the expandable element (e.g., a balloon) is inflated.
The balloon structure also includes a circumferential ablation element, e.g., an RF electrode carried on the outer surface of the balloon, which performs the ablation procedure. In order for the electrode to be effective, it must in contact with the tissue throughout the procedure. In such devices, a major limitation in prior art percutaneous designs is their inability to maintain such contact with the actual and quite varied geometry of the heart. The inner surface of the atrium is not regular. In particular, the mouths of the pulmonary veins do not exhibit regularity; they often bear little resemblance to conical or funnel-shaped openings. When the expandable, contact heating devices of the prior art encounter irregularly-shaped ostia, the result can be an incompletely formed (non-circumferential) lesion. This limitation is also present in the case of cryogenic balloon catheters where contact with the target tissue is required in order to perform the desire lesion.
Another problem commonly encountered in maneuvering an instrument within the left atrium is the need to detect side branches in the pulmonary veins. If the energy is delivered into a side branch a lesion will not be formed at that location and a circumferential lesion will not be formed. A related problem is that it is often difficult to distinguish between blood at the target site, which can sometimes be remedied by reseating of the instrument, and the presence of a vessel side branch, which will preclude formation of a continuous lesion regardless of attempts to reseat at the location. This problem is underscored by a study by Saliba et al., Journal of Cardiovascular Electrophysiology, Vol. 13, No. 10, pp. 957-961 Oct. 12, 2002, in which the authors report a success rate of only 39% in treating atrial fibrillation in 33 patients using a circumferential ultrasound ablation device. Among the causes for failure of the device suggested by the authors were eccentric balloon placement in the pulmonary vein and inability to detect early branching of the vein which would result in ineffective circumferential energy delivery.
Another limitation in the prior art percutaneous designs is the lack of site selectability. Practically speaking, each prior art percutaneous instrument is inherently limited by its design to forming an ablative lesion at one and only one location. For example, when an expandable balloon carrying an RF heating surface on it surface is deployed at the mouth of a vein, the lesion can only be formed at a location defined by the geometry of the device. It is not possible to form the lesion at another location because the heating element must contact the target tissue. If the ablation element is not in contact with tissue at some location the entire catheter must be repositioned in an attempt to obtain contact. Often it is not possible to find a single position where contact is obtained completely around the vein.
U.S. Pat. No. 6,514,249 issued to Maguire et al. on Feb. 4, 2003 discloses one approach to positioning an ablation element within a pulmonary vein ostium. This patent describes the use of ultrasound, pressure or temperature sensors to determine if the instrument is in contact with a target region of tissue. While such sensors can be useful in determining contact at a particular location, the devices disclosed by U.S. Pat. No. 6,514,249 can not provide an image of the anatomy of the vein, the location of side branches or the position of the instrument relative to the target site in a manner that would allow a clinician to devise an optimal ablation plan.
Accordingly, there is a need for better ablation instruments that can form lesions with less trauma to the healthy tissue of the heart and provide a greater likelihood of successfully producing fully circumferential lesions. A percutaneous system that can aid in visualizing the anatomy of the vein, in determining the location of side branches, in visualizing the position of the catheter relative to the vein and any side branches, in determining whether contact has been achieved (or blood has been cleared from an ablative energy transmission path) before ablation, and in determining the optimum location for energy delivery would improve the likelihood of first-time success based on such determinations and would represent a significant improvement over the existing designs.
Moreover, a percutaneous ablation device that allowed the clinician to select the location of the ablation site based upon such visualization would be highly desirable. One such device is described in commonly-owned, parent applications, e.g., U.S. patent application Ser. No. 10/357,156, filed Feb. 3, 2003 and Ser. No. 09/924,393, filed on Aug. 7, 2001, which disclose a balloon ablation device having an energy emitting element and an optical sensor. The energy emitting element is independently positionable within the lumen of the instrument and adapted to project ablative energy through a transmissive region of a balloon to a target tissue site.
Because the position of the radiant energy emitter can be varied, the clinician can select the location of the desired lesion without the necessity of repositioning the instrument. Applicants' basic devices, as described in the prior filings, thus, allow a clinician to choose from a number of different lesion locations and, if desired, also permit the formation of composite lesions that combine to form continuous conduction blocks around pulmonary veins or other cardiac structures.
However, even Applicants' advanced ablation devices could benefit from enhanced visualization of the cardiac anatomy. Similarly, any sophisticated cardiac ablation device could likewise benefit from visual data in instrument positioning, identifying targets, and/or monitoring the progress of ablative procedures as well as selecting an optimal dose based on the location to be treated.